THE PLANETS AND OUR SOLAR SYSTEM

All of the planets and moons in our solar system emit strongly in the infrared.
This infrared emission is the heat from atmospheres and surfaces which peaks in
the mid to far infrared (15 to 100 microns). Solar system objects also reflect
infrared radiation from the Sun. This reflected radiation peaks in the near
infrared at about 0.5 microns.
The study of the infrared radiation from solar system objects has given us much
information about their composition.
For planets and moons which have atmospheres, infrared studies can show us
how the abundance and composition of atmospheric gases as well as the
temperature of the atmosphere vary with depth.
In addition, infrared astronomy has led
to the discovery of new comets and asteroids and bands of dust which lie in
our solar system.

VENUS:
Sunlight passing through the atmosphere of Venus is absorbed by its surface
which causes it to heat up, giving off radiation which is primarily infrared.
The carbon dioxide in Venus's thick atmosphere traps much of the infrared heat,
giving Venus a surface temperature of 750K (890 degrees Fahrenheit) which
is hot enough to melt lead.
By studying the infrared spectra of Venus, astronomers found that sulfuric acid
droplets exist in its atmosphere.
Compare the
infrared image of the clouds of
Venus on the left with the
visible light image of Venus on the right. Both images
were taken by the Galileo satellite.

JUPITER:
Jupiter radiates ~1.6 times as much heat, in the form of infrared
energy, as it receives from the Sun. This indicates that Jupiter has an
internal source of energy - probably heat created by Jupiter's collapse when
it was formed.
By studying the infrared emission from Jupiter we have learned much about
its cloud structure. Jupiter's belts (its dark horizontal bands) are
brighter in the infrared than its zones (its bright horizontal bands)
This indicates that the belts are regions of hotter gas. The
temperature of Jupiter also increases towards its center, so the zones
are at higher levels in Jupiter's atmosphere than the bands.
To the left is an
infrared image of Jupiter and one of its moons, Io
(the bright spot on the image). Io has several
active volcanoes and is heated by tidal forces caused by the gravitational
pull of Jupiter and the other Galilean moons.

SATURN:
Like Jupiter, Saturn also radiates about twice as much radiation
in the infrared as it received from the Sun. ISO was recently used to study
the concentrations of heavy and ordinary
hydrogen in the atmosphere of Saturn. These measurements give information
about the composition of the original cloud of gas and dust from which the
sun and planets formed.
The
HST/NICMOS infrared image of Saturn below shows the details of its cloud structure.

Credit: NASA/AURA/STScI

TITAN:
In 1944, Gerard Kuiper discovered that Titan, the largest moon of Saturn,
had an atmosphere. He detected the methane in Titan's atmosphere by
studying its infrared emission. In 1994, astronomers using the Wide Field
Planetary Camera on the Hubble Space Telescope made the first images of the
surface of Titan. These images were made in the near-infrared since infrared
radiation is able to penetrate the hazy atmosphere of Titan. Titan is larger than the
planet Mercury and only a bit smaller than Mars. The infrared images of Titan's
surface show a bright area which is a surface feature that is about 2,500 miles
across (about the size of Australia). The left image below is a set of four
near infrared pictures of Titan. Compare this to the
visible light image of Titan on the right taken by Voyager 2.
Both images are supplied by NASA and the NSSDC.

ASTEROIDS:
Asteroids are rocky-metallic objects which range in size from
about 1 to 1000 km. They orbit the Sun and
are thought to be leftover material from the formation of the planets in
our solar system. Most of the asteroids are found in the Asteroid Belt
which lies between the orbits of Mars and Jupiter. Astronomers have also
identified a group of asteroids whose orbits cross Earth's orbit. The
infrared radiation from an asteroid
can be used to get information about its location,
composition, rotation and its shape and size.
The IRAS mission
discovered over 400 new asteroids and also provided infrared data on about 1800
previously known asteroids.

COMETS:
Comets are basically dusty snowballs which orbit the Sun. They consist of
an icy nucleus surrounded by a large cloud of gas and dust (called the coma).
The coma is created as the ice in the nucleus is warmed and vaporizes.
Comets have 2 tails, a straight gas tail and a curved dust tail.
The gas tail is created by the solar wind whose magnetic fields pull the gas
away from the comet's coma. The dust in the coma is not affected by
magnetic fields but is pushed out by the Sun's radiation. The dust in the
tail reflects sunlight and radiates in the infrared.
The infrared emission from comets can be used to get information on the nature
of the dust they contain as well as on the rate at which material is being lost
from the nucleus.
Using IRAS, astronomers discovered that dust from comets fills the Solar System
and that comets are dustier than they were thought to be. Many of the meteors
which are seen as they streak through our atmosphere may be the larger pieces
of this comet dust.
IRAS was the first satellite to discover a comet (comet IRAS-Araki-Alcock).
In all, IRAS discovered 6 new comets and collected infrared data on 25
previously known comets.

COMET IMPACT WITH JUPITER:
In 1994 several fragments of comet Shoemaker-Levy 9, which had broken apart
during its previous orbit in 1992, collided with the planet Jupiter. These
impacts released a tremendous amount of energy into Jupiter's atmosphere.
Spectacular images of comet
Shoemaker-Levy's collision with Jupiter were taken in the infrared.
The first image shows the
impact of fragment A
(Courtesy of NASA/NSSDC,
Tom Herbst, Max-Planck-Institut fuer Astronomie, Heidelberg, Doug Hamilton,
Max-Planck-Institut fuer Kernphysik, Heidelberg, Hermann Boehnhardt,
Universitaets-Sternewarte, Muenchen, and Jose Luis Ortiz Moreno,
Instituto de Astrofisica de Andalucia, Granada)
The second image is an infrared color composite showing fragments A,E
F,G and H impacting Jupiter (Courtesy of NASA/NSSDC).

ZODIACAL DUST BANDS:
IRAS also discovered bands of infrared emission that girdle our solar system.
Called the zodiacal dust bands, these are likely to be debris from
colliding asteroids. Two bands appear 9
degrees above and 9 degrees below the ecliptic (the plane formed by the
Earth's orbit around the Sun), which result from debris in an orbit
about the sun that is inclined by 9 degrees to the ecliptic. The bands
result from the particles spending more time at the extremes of their
orbit, causing an apparent increase of density at plus and minus 9
degrees. Another one is found in the ecliptic plane.
The infrared emission of these bands show a temperature of 165-200K (or -163 to -100 degrees Fahrenheit) and a
distance of 2.2 - 3.5 A.U. from the Sun (1 A.U. is the distance from the Earth to the
Sun which is about 93 million miles. A.U. stands for Astronomical Unit)
This places these dust bands between Mars and Jupiter in the
region of the asteroid belt.
IRAS has found evidence of zodiacal dust bands around other
stars as well.